In a paraffinic froth treatment (PFT) process, bitumen froth is mixed with paraffinic solvent and separated into an underflow component containing precipitated asphaltenes, water, fine solids and residual paraffin and bitumen, and an overflow component which may be referred to as high diluted bitumen. This high diluted bitumen contains paraffinic solvent and water, which it is desirable to remove in order to produce dry bitumen for upgrading or pipelining to other markets.
More particularly, the production of dry bitumen from a PFT process requires removal of high quantities of paraffinic solvent from the high diluted bitumen containing asphaltenes at the precipitation/solubility limit as well as water largely dissolved in the hydrocarbon phase.
Conventional solutions for treating the high diluted bitumen adapt diluent recovery processes that were developed and applied in conventional naphthenic froth treatment processing, for use in paraffinic froth treatment. These processes involve heating, stage flashing and fractionate to recover diluent and feed downstream upgrading operations such as vacuum fractionation of a gas oil product or coking or hydrocracking. These downstream upgrading operations require high feed temperatures.
The high diluted high bitumen produced by PFT differs from diluted bitumen from conventional naphthenic froth treatment. First, conventional froth treatment produces a diluted bitumen product with diluent/bitumen (D/B) ratios on a wt/wt basis of 0.45 to 0.8. Single stage flash vessels are typically used to reduce diluent to levels acceptable for subsequent processing and fractionation. For paraffinic froth treatment, the diluted bitumen product has D/B ratios ranging from 1.2 to 2.5 wt/wt and is termed high diluted bitumen. As flash vessel sizing to minimize entrainment and carryover of feed droplets depends directly on the vapour velocity, the high diluent loads in high diluted bitumen derived from PFT require large diameter flash vessels. Secondly, in PFT, high diluted bitumen contains asphaltenes in equilibrium with the paraffinic solvent at the temperature and, to a lesser extent, the pressure of the froth settling vessel from which it overflows. To flash diluent requires inputting heat to the stream. However, the solubility of asphaltenes in paraffinic solutions does not increase linearly with temperature. Consequently as the stream is heated, asphaltenes precipitate from saturated solutions causing equipment to foul. The asphaltene fouled equipment normally must be removed from service for cleaning and restoration of equipment performance Third, the limited understanding of the equilibrium between paraffinic solvents and entrained bitumen with asphaltenes has limited enhanced design and operation of solvent recovery processes for high diluted bitumen. At high solvent concentrations in overhead systems, asphaltenes in bitumen entrained from flash separators precipitate and foul piping and equipment. At high solvent recoveries, the maltene fraction of the bitumen can selectively accumulate in the solvent and adversely affect the separation of bitumen from bitumen froth. Conventional techniques have not been able to minimize entrainment for reliable plant operation. Fourthly, process temperatures in diluent recovery plants for conventional froth treatment process are 200° C. to 275° C. for atmospheric flashing (100-200 kPa) to separate naphtha diluent from diluted bitumen. At these operating conditions, water in naphtha diluted bitumen flashes and is condensed for separation in the overhead diluent separator. The diluent solvents used in PFT are more volatile and, consequently, can separate from bitumen at lower process temperatures. The lower energy requirement for these temperatures is constrained by water flashing and condensing within a similar pressure and temperature, resulting in unstable separation in column operations. Fifthly, unit operations have viewed the froth treatment separation as distinct and separate from diluent recovery plant with surge tankage between unit operations. To provide for surge capacity between the unit operations the tankage has been large. Also, limiting emissions by the volatile diluent solvent have required cooling run down streams to tankage and heating return fluids to the process temperature from tankage. Sixthly, process equipment for heating the diluted bitumen conventionally has not had to deal with large vapour loads, as the naphthenic diluent has a large boiling point range and the diluent recovered is lower in ratio to the bitumen processed. The paraffinic process has larger vapour loads and has a very narrow boiling point.
As more general background on PFT in the context of oil sands processing, extraction processes are used to liberate and separate bitumen from oil sand so the bitumen can be further processed. Numerous oil sand extraction processes have been developed and commercialized using water as a processing medium. One such water extraction process is the Clarke hot water extraction process, which recovers the bitumen product in the form of a bitumen froth stream. The bitumen froth stream produced by the Clarke hot water process contains water in the range of 20 to 45%, more typically 30% by weight and minerals from 5 to 25%, more typically 10% by weight which must be reduced to levels acceptable for downstream processes. At Clarke hot water process temperatures ranging from 40 to 80° C., bitumen in bitumen froth is both viscous and has a density similar to water. To permit separation by gravitational separation processes, commercial froth treatment processes involve the addition of a diluent to facilitate the separation of the diluted hydrocarbon phase from the water and minerals. Initial commercial froth treatment processes utilized a hydrocarbon diluent in the boiling range of 76-230° C. commonly referred to as a naphtha diluent in a two stage centrifuging separation process. Limited unit capacity, capital and operational costs associated with centrifuges promoted applying alternate separation equipment for processing diluted bitumen froth. In these processes, the diluent naphtha was blended with the bitumen froth at a weight ratio of diluent to bitumen (D/B) in the range of 0.3 to 1.0 and produced a diluted bitumen product with typically less than 4 weight percent water and 1 weight percent mineral which was suitable for dedicated bitumen upgrading processes. Generally, operating temperatures for these processes were specified such that diluted froth separation vessels were low pressure vessels with pressure ratings less than 105 kPag. Other froth separation processes using naphtha diluent involve operating temperatures that require froth separation vessels rated for pressures up to 5000 kPag. Using conventional vessel sizing methods, the cost of pressure vessels and associated systems designed for and operated at this high pressure limits the commercial viability of these processes.
Heavy oils such as bitumen are sometimes described in terms of relative solubility as comprising a pentane soluble fraction which, except for higher molecular weight and boiling point, resembles a distillate oil; a less soluble resin fraction; and a paraffinic insoluble asphaltene fraction characterized as high molecular weight organic compounds with sulphur, nitrogen, oxygen and metals that are often poisonous to catalysts used in heavy oil upgrading processes. Paraffinic hydrocarbons can precipitate asphaltenes from heavy oils to produce deasphalted heavy oil with contaminate levels acceptable for subsequent downstream upgrading processes. Contaminants tend to follow the asphaltenes when the asphaltenes are precipitated by paraffinic solvents having compositions from C.sub.3 to C.sub.10 when the heavy oil is diluted with about 2 to 10 times the volume of solvent.
High water and mineral content distinguish bitumen froth from the heavy oil deasphalted in the above processes. Some early attempts to adapt deasphalting operations to processing bitumen from oil sands effected precipitation of essentially a mineral free, deasphalted product, the ability to vary the amount of asphaltene precipitated, and the enhancement of asphaltene precipitation by addition of water and chemical agents.
Recent investigations and developed techniques in treating bitumen froth with paraffinic use froth settling vessels (FSV) arranged in a counter-current flow configuration. In process configurations, counter-current flow refers to a processing scheme where a process medium is added to a stage in the process to extract a component in the feed to that stage, and the medium with the extracted component is blended into the feed of the preceding stage. Counter-current flow configurations are widely applied in process operations to achieve both product quality specifications and optimal recovery of a component with the number of stages dependent on the interaction between the desired component in the feed stream and the selected medium, and the efficiency of stage separations. In deasphalting operations processing heavy oil with low mineral solids, separation using counter-current flow can be achieved within a single separation vessel. However, rapidly setting mineral particles in bitumen froth preclude using a single separation vessel as this material tends to foul the internals of conventional deasphalting vessels.
A two stage paraffinic froth treatment process is disclosed in Canadian Patent No. 2,454,942 (Hyndman et al.) and represented in its FIG. 1 as a froth separation plant. In a froth separation plant, bitumen froth at 80-95° C. is mixed with overflow product from the second stage settler such that the solvent to bitumen ratio in the diluted froth stream is above the threshold to precipitate asphaltenes from the bitumen froth. For paraffinic froth treatment processes with pentane as the paraffinic solvent, the threshold solvent to bitumen ratio as known in the art is about 1.2 which significantly increases the feed volume to the settler. The first stage settler separates the diluted froth into a high dilute bitumen stream comprising a partially to fully deasphalted diluted bitumen with a low water and mineral content, and an underflow stream containing the rejected asphaltenes, water, and minerals together with residual maltenes from the bitumen feed and solvent due to the stage efficiency. The first stage underflow stream is mixed with hot recycled solvent to form a diluted feed for the second stage settler. The second stage settler recovers residual maltenes and solvent to the overflow stream returned to the first stage vessel and froth separation tailings. It is important to recognize the different process functions of stages in a counter-current process configuration. In this case, the operation of first stage settler focuses on product quality and the second stage settler focuses on recovery of residual hydrocarbon from the underflow of the first stage settler.
The process may be operated at temperatures that require controlling the pressure in either settler stage to limit solvent vaporization. The concentration of solvent in diluted bitumen and temperature for a specific paraffinic solvent such as pentane determine the solubility and hence the rejection of asphaltenes. While low asphaltene rejection maximizes bitumen recovery, the asphaltene content may limit processing options in upgrading operations particularity those based on hydrogen addition.
The diluent recovery from diluted bitumen produced by conventional froth treatment closely resembles conventional crude oil distillation (see for example Andrews et al. “Great Canadian Oil Sands Experience in Commercial Processing of Athabasca Tar Sands” American Chemical Society San Francisco Meeting Apr. 2-5 1968) in that diluent is recovered as an overhead product and the bitumen as a bottom product is fed to cokers at 260° C. for upgrading. Relative to diluted bitumen for conventional froth treatment, the conventional diluent recovery encounters a number of problems in processing high diluted bitumen produced by paraffinic froth treatment processes.
The naphtha diluents are composed of various hydrocarbons resulting in atmospheric pressure boiling temperatures ranging from 76° C. up to the initial boiling point of 230° C. for bitumen. With this boiling range, high diluent recoveries require high distillation temperatures for the diluent-bitumen separation. However, using a specific paraffinic solvent range of specific paraffins such as pentanes as a diluent have a narrow boiling range: pentanes for example boil at about 28-36° C. With this narrow boiling range, flashing of paraffinic diluent to the vapour phase is sensitive to variations in operating pressures and can result in excessive entrainment of bitumen droplets. In addition, the boiling point of water 100° C. is between the diluent and bitumen boiling ranges and can adversely affect the stability of the distillation in producing a dry bitumen product that can be marketed to upgraders remote to the froth treatment plant.
The naphtha diluents dilute bitumen to permit gravitational separations of water and mineral from the hydrocarbon phase without significant precipitation of asphaltenes. However, paraffinic froth treatment processes use paraffinic diluent to reject residual water and minerals with partial rejection of asphaltenes and produce high diluted bitumen comprising asphaltenes determined by operating temperature and the solvent to bitumen ratio in the froth separation vessel. The solubility of asphaltenes in high diluted bitumen creates two notable problems for diluent recovery. Firstly, the solubility of asphaltenes in high diluted bitumen depends on temperature and as temperature increases for distilling diluent, asphaltenes can precipitate with deposits fouling equipment. Secondly, bitumen droplets entrained into overhead diluent streams precipitate asphaltenes at the high paraffinic concentrations that foul overhead systems and the maltenes fraction of the bitumen adversely affect froth separation by increasing solubility of asphaltene.
Conventionally, diluent recovery has been viewed as part of conventional refining operation with some heat integrated across the refinery. Tankage between the froth treatment and diluent recovery can allow each operation to operate independently. However, paraffinic froth treatment operations may operate independently from refinery operations and, consequently, high levels of heat may be potentially lost from the paraffinic froth treatment process.
It is clear that the known techniques and methods of treating high diluted bitumen in a PFT process have several drawbacks and shortcomings